Conventionally, an object detection apparatus for detecting an object, or existence of an object, by transmitting a pulse signal and receiving a reflected signal reflected by an object is known. In such an apparatus, for the purpose of improving an object detection distance, it is necessary to improve a signal to noise ratio (S/N ratio) of the received signal.
In view of the above, Japanese patent No. 4,283,170 (JP '170) (U.S. Pat. No. 7,496,157) discloses an apparatus which improves the S/N ratio of the received signal by using a technique of so-called “pulse compression.” According to the apparatus of JP '170, a modulation signal made by a frequency modulation or a phase modulation of the pulse signal is transmitted, and, after receiving the reflected wave, the received reflected wave is orthogonally-demodulated (i.e., quadrature de-modulation) to make a received signal. Then, correlation (i.e., auto-correlation) between the received signal and the modulation signal is calculated, and such calculation result is used to compress the pulse width of the received signal, for the improvement of the S/N ratio of the received signal which is generated from the reflected wave of a standstill object.
Further, JP '170 discloses an object detection apparatus that uses a correction signal for correcting the Doppler shift of the received signal, which is received as a Doppler shifted signal from an object that moves at a constant speed (i.e., a uniform-motion object). In such an apparatus, the improvement of the S/N ratio of the received signal generated from the reflected wave of the uniform-motion object is achieved by calculating correlation between the modulation signal and the received signal after the Doppler shift correction.
Furthermore, JP '170 discloses an object detection apparatus that improves the S/N ratio of the received signal that is generated from the reflected wave of the standstill object, by (i) calculating a frequency of the Doppler shift of the received signal generated from the reflected wave of the standstill object based on the speed of the vehicle in which the object detection apparatus is installed and (ii) changing a correction signal according to the calculated frequency.
When the object detection apparatus is installed on a moving body such as a vehicle, it may be required for the apparatus to detect various objects having respectively different relative speed ranges, relative to the moving object. For example, if the moving object is a vehicle (i.e., a subject vehicle), and, if the subject vehicle is traveling on an expressway, a detection object detected by the object detection apparatus may only be other vehicles that travel side-by-side with the subject vehicle (i.e., other vehicles with relatively small speed ratio against the subject vehicle). If the subject vehicle is in a parking facility, in which the subject vehicle travels at a low speed, the relative speed of the detection object may vary in a wide range, because the detection object may be a bumping post, which is a standstill object, or may be a walker, which is a moving object. Such objects in various relative speed ranges may have to be detected at the same time.
In the technique of JP '170, the object detection on the moving body such as vehicles performs correction of the Doppler shift of the received signal, which is generated from the reflected wave of the moving object which is moving relative to the moving body, and then performs the pulse compression, for the purpose of improving the S/N ratio of the received signal. However, it is not mentioned in JP '170 that the range of the relative speed of the detection object is changed according to the speed of the moving body which is equipped with the object detection apparatus. Therefore, as for the object detection apparatus disclosed in JP '170, it is a problem that the range of the relative speed of the detection object cannot be changed according to the speed of the moving body when the object detection apparatus is installed in the moving body such as a vehicle.